Polychlorinated biphenyls (PCBs) were widely used and are considered now as one of the most problematic pollutants. The most promising approach for bioremediation of PCBs is a combined anaerobic-aerobic process. Anaerobic degradation results in reductive dechlorination of highly chlorinated PCBs and accumulation of less chlorinated congeners. Eight congeners including 2-, 4-, 2,4-, 2,6-, 2-2-, 2-4-, 2,4-2-, and 2,4-4-chlorobiphenyl (CB) comprise from 70 to 85% of anaerobic dechlorination products from Aroclor 1242. Although many aerobic bacteria have been screened for the spectra of PCBs which they can degrade, little is known about degradation and final products formed from congeners that accumulate during anaerobic dechlorination processes. Four bacterial strains were chosen for this investigation based on their superb PCB degradative abilities. Mass spectrometry was employed to analyze the specificity of the primary PCBs attack and the final products formed by aerobic bacteria. Based on mass spectrometry, UV-spectroscopy and HPLC data strains Rhodococcus sp. RHA1 and Burkholderia sp. LB400 were concluded to preferentially attack on ortho-chlorinated rings, while another two strains (Commamonas testosteroni VP44 and Rhodococcus sp. NY05) preferentially oxidized a para-substituted rings. It was shown that the ortho-focused strains showed more efficient degradation of Aroclor anaerobic dechlorination products. It is known that aerobic degradation of PCBs usually involves partial degradation and results in the production of chlorobenzoates. We also found chlorobenzoates as the major products of degradation of selected congeners. However, degradation of 2-4 and 2,4-4-CB by para-focused bacteria resulted in incomplete transformation of these congeners and accumulation of meta-cleavage products of these chlorobiphenyls as a dead end metabolites. These compounds were identified based on mass spectrometric data (GC-MS).